It is well known in the color photography art that color images are produced by a colored dye which is formed by a coupling reaction between an oxidized product of an aromatic primary amine color developing agent and a coupler. Various types of cyan, magenta and yellow dye-forming couplers are well known for use in such coupling reactions. For example, a coupler of the acylacetanilide or benzoylacetanilide type may be used for forming a yellow dye image, a coupler of the pyrazolone, pyrazoloazole, pyrazolobenzimidazole, cyanoacetophenone or indazolone type may be used for forming a magenta dye image, and a coupler of the phenolic or naphtholic type may be used for forming a cyan dye image.
It is also known in the art that the coupler may be in the form of a polymer which improves the ability of the dye to remain in the location where it is formed in a color photographic material. For example, the Ponticello et al U.S. Pat. No. 4,215,195, the Lau et al U.S. Pat. No. 4,612,278 and the Maekawa et al U.S. Pat. No. 4,946,771 disclose various silver halide photographic materials including polymeric couplers. The Umberger U.S. Pat. No. 3,451,820 and the Hirano et al U.S. Pat. Nos. 4,511,647 and 4,518,687 disclose further silver halide color photographic materials which employ cyan dye-forming polymer couplers formed from a mixture of ethylenically unsaturated monomers.
One difficulty encountered with the use of polymeric couplers in photographic materials is in providing good dispersions of the polymers which allow incorporation of the polymers in the photographic material. Generally, three methods have been employed in the past for dispersing polymeric couplers. In the first method, the polymeric coupler is formed by solution polymerization and is isolated by precipitation from a poor solvent. As is known in the polymer art, solution polymerization employs a solvent as the reaction medium in a homogeneous system, [Billmeyer, Jr., Textbook of Polymer Science, Wiley-Interscience (1971)]. The resulting solid polymer coupler is then dissolved in ethyl acetate, with or without additional coupler solvents, mixed with gelatin and surfactants, and passed through a colloid mill to produce a fine dispersion. However, this method is disadvantageous in that it requires many energy-consuming steps.
A second method for dispersing polymer couplers requires the incorporation of a large amount, usually greater than about 40 weight percent, of ionic monomers in the polymeric coupler. The resulting polymers are water-soluble and can be directly mixed with gelatin and coated with a silver halide emulsion on a support. However, this method is disadvantageous in that the use of the relatively large amount of ionic monomers increases the equivalent weight of the polymers, results in gel-polymer interactions and causes increased wandering.
A third method for dispersing polymeric couplers comprises the formation of water-dispersible polymers by emulsion polymerization or suspension polymerization as taught, for example, in the Monbaliu et al U.S. Pat. No. 3,926,436, the Van Paesschen et al U.S. Pat. No. 4,080,211, the Yagihara et al U.S. Pat. No. 4,474,870, British Reference No. 2,092,573 and European Pat. Application No. 321,399. The method employing emulsion polymerization is usually preferred. However, such methods are disadvantageous in several respects. That is, owing to the low solubility of many coupler monomers in water, organic solvents are required in the emulsion polymerization. This tends to reduce the stability of the resulting polymer latexes and to reduce the percentage of solids in the polymer products. Additionally, the compositions of the polymeric couplers made by emulsion polymerization are difficult to control because of the heterogeneous nature of the system. Coupler monomers usually are solid and owing to their low water solubility, they tend to precipitate out in the aqueous phase and fail to copolymerize with other comonomers. Finally, polymeric couplers made by emulsion polymerization methods can not be isolated and redispersed in water.
Thus, a need exists for novel polymeric couplers which may be employed more easily and effectively in color photographic materials.